On Wed, Jun 10, 2015 at 05:04:12PM +0200, Andi Kleen wrote: > > > > - duplicate the destination code inside the function > > > > - convert the jump to a call > > > > > > That all won't work for a lot of cases. > > > > Hm, could you give an example? > > Just a standard *_user exception handler. I'm afraid I don't follow. Exception handlers don't work via jump instructions, but rather via CPU exceptions. Or are you talking about something else? > > Well, I don't see how that's really a logical conclusion. > > What's special about assembler code? Ok, I'll bite: - it's human-generated - it's much simpler - it doesn't have stack metadata by default - it has far fewer constraints But I don't see this line of discussion going anywhere without any real examples of why you need external jumps in asm functions... > > But we're > > probably being too vague here... Do you have any examples where you > > really need to jump outside of a callable function? > > It's not needed, but it's an optimization to optimize icache usage. > It is optional (-freorder-blocks-and-partition) > > In this case gcc splits the function into two (hot and cold) > > It's actually a nice optimization and it would be sad from stopping > the kernel from using it. Sorry if I wasn't clear, I was trying to ask for examples in kernel asm code. Are you suggesting that we implement this gcc optimization in kernel asm code? -- Josh -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
